The Functional Morphology of Cleaning

Larabicus quadrilineatus

Cleaner fishes are expected to possess traits that enable them to detect, remove, and consume ectoparasites. I am interested in understanding how prey capture occurs in these species, as such information is crucial to understanding trait evolution in cleaners.

I have used high-speed videography (1000 fps) to record and analyze the feeding behaviors of juveniles in three species of cleaner wrasses: Labroides dimidiatus, Larabicus quadrilineatus, and Thalassoma lutescens. I recently published some of my findings in Journal of Morphology (Baliga and Mehta, 2015). This study paired kinematic findings with dissections of the cranial skeleton of each species. To my knowledge, this is the first documented analysis of the kinematics of feeding in cleaner fishes.

Cranial skeleton of Thalassoma lutescens. Three sections of the adductor mandibulae complex (a muscle that functions in jaw closing) are superimposed.

Still image from a high-speed video of Labroides dimidiatus inspecting a suspended (euthanized) client fish. This cleaner can feed upside down!

I discovered that cleaning in these species is associated with low-displacement, fast jaw movements that allow for rapid gape cycles on individually-targeted items. In all three species, prey were captured predominately via biting, using only the anterior tips of the jaws. In particular, the curve of the alveolar process of the premaxilla in Labroides dimidiatus and Larabicus quadrilineatus ensures that only the anteriormost teeth make contact with prey -- akin to using a pair of forceps.

That is not to say that this feeding behavior is accomplished by biting alone. In fact, suction plays an important role in prey capture in all three species. I found that the force exerted via suction generation often aids in orienting the prey towards the buccal cavity of the cleaner. In cases where prey were weakly attached to a substrate, suction alone was sufficient for prey capture and biting may not occur.

Regardless of the kinematic pattern of jaw movement (biting + suction vs. suction alone), the jaws of these cleaners show extremely small displacements. Not only do cleaners show little jaw rotation and small vertical gapes, but also the magnitude of jaw protrusion is quite small. These small excursions and rotations are coupled with some of the fastest jaw movements seen in wrasses.